This invention pertains generally to a device for sensing, measuring and recording fluorescence emitted by certain materials in a scene viewed in sunlight. More particularly, the present invention relates to an apparatus which measures fluorescent radiation emitted by certain materials at unique ground points in a narrow, preselected wavelength.
The basic theory employed in Fraunhofer line discrimination is as follows. The solar spectrum contains a number of very sharp (Fraunhofer) absorption lines created in the photosphere of the sun. When sunlight is reflected, some wavelengths tend to be reflected more than others, but the variation of reflectivity with wavelength is relatively smooth across the solar spectrum. Consequently, the signature Fraunhofer lines are retained in the reflected radiation. The same is not true in the presence of fluorescence, which is characterized by a strong absorption in one spectral region followed by re-emission at longer wavelengths. Provided the bandwidth of these re-emissions encompass a Fraunhofer line wavelength they tend to remove the sharp Fraunhofer lines, again yielding a relatively smooth emission spectrum. Any addition of a fluorescence spectrum to a reflected solar spectrum therefore tends to reduce the relative depth of the Fraunhofer absorption lines near the fluorescence emission peak. Detection of this reduction of the relative depth of Fraunhofer lines yields detection of fluorescence in the scene being viewed.
Various techniques have been developed to take advantage of this phenomenon to measure fluorescence within a sunlit scene. Some of these techniques are disclosed in U.S. Pat. No. 3,769,516 which is the basis for a successful, but limited capability, Fraunhofer line discriminator (FLD). Other techniques, more similar to the present invention, are disclosed in U.S. Pat. No. 3,598,994. Both U.S. Pat. Nos. 3,769,516 and 3,598,994 have the same assignee as the present invention.
The FLD disclosed in U.S. Pat. No. 3,769,516 provides a sky telescope and an earth telescope, each of which forms radiation beams which are directed to a single optical chopper. The chopper sequentially directs each beam through a single Fabry-Perot filter centered on a Fraunhofer line and a single neutral density filter. The chopped segments are recombined to form a beam with four components: sky-looking radiation inside and outside the Fraunhofer line and earth-looking radiation inside and outside the Fraunhofer line. A single photomultiplier tube, with a blocking filter in front, receives the beam and produces corresponding sequential electrical pulses which are processed to yield fluorescence level in the scene being viewed.
While the apparatus disclosed in the U.S. Pat. No. 3,769,516 was useful in most applications it did not meet the higher performance requirements of the present generation of Fraunhofer line discriminators (FLD's). Such requirements for FLD's are enhanced sensitivity or signal-to-noise ratios, improved spatial resolution and wider coverage.
In U.S. Pat. No. 3,598,994 a bundle of sunlight is encoded and split onto two beams. Each of these beams is combined with similar beams, also, encoded, which contain solar reflected and solar excited fluorescent radiation from a scene being viewed. The two beams (each of which contain reference and sample components) are passed through two spectral filters and detected photo electrically on two separate detectors. One of the spectral filters is centered on a Fraunhofer absorption line, the other is centered a few Angstroms away in the solar continuum. The four signals, corresponding to the direct solar intensity inside and outside the Fraunhofer line and the intensity of radiation of the scene being viewed inside and outside the Fraunhofer line, are separated electronically. These four signals are then combined in an analog computer to yield a signal proportional to the fluorescence of the sample material.
While the device disclosed in the U.S. Pat. No. 3,598,994 did advance the art in this area, actual use proved difficult due to problems with the photo electric detectors. Further, the device only produces a signal indicating presence and amount of fluorescence in a scene being viewed. It does not indicate the location of fluorescent materials in a given scene nor does it indicate variations of fluorescence over different portions of the scene.
Accordingly, the present invention provides an improved Fraunhofer line discriminator which has enhanced spatial resolution, discrimination and high coverage. The manner in which the foregoing objects are achieved will be apparent from the following description and appended claims.